This invention relates generally to the field of marine electromagnetic surveying, and, at least in some embodiments, to novel configurations of electromagnetic sources and receivers which may provide opportunities to more efficiently capture, process, and interpret geophysical data.
Marine electromagnetic surveying may be used to identify hydrocarbon reserves in formations below the bottom of a body of water, such as a lake or ocean. In a typical marine electromagnetic survey, an electromagnetic (“EM”) source and a number of EM receivers are located in a body of water. The EM source may be towed by a survey vessel over a formation of interest, and the receivers may be disposed over the formation, often at or near the bottom of the body of water, to obtain signals related to the distribution of EM resistivity in the formation. The receivers may also be towed by the same or other survey vessel. Such surveying is typically performed for a range of EM source and EM receiver positions. The EM source may emit either or both a time varying electric field and a time varying magnetic field, which may propagate outwardly into the overlying water and downwardly into the formation below the water. The time varying EM field may be induced by passing electric current through an antenna. The electric current may be continuous wave and have one or more discrete frequencies. It is also known in the art to apply direct current to an antenna, and produce transient EM fields by switching the current. Such switching may include, for example, switching on, switching off, inverting polarity and inverting polarity after a switch on or switch off event. Such switching may be equally or unequally time spaced, in a time series known as a pseudo random binary sequence (“PRBS”), or in some other selected sequence.
EM energy may be rapidly attenuated in seawater, but less conductive subsurface formations attenuate EM energy less, allowing it to propagate more efficiently. Additionally, hydrocarbon-containing subsurface reservoirs tend to be more resistive than permeable formations with aqueous saline fluids. If the frequency of the EM energy is low enough, it can propagate deep into subsurface formations. Energy may be reflected from subsurface layers back to the water bottom. When the source-receiver spacing (“offset”) is comparable-to or greater-than the formation depth of the subsurface layer (the depth below the water bottom), the reflected energy will dominate over the transmitted energy. Marine electromagnetic surveying uses the large resistivity contrast between hydrocarbon-containing subsurface reservoirs and permeable formations with aqueous saline fluids to assist in identifying the geophysical properties of the formation.
Marine electromagnetic surveying typically requires a minimum of four electrodes: two source electrodes through which a current is passed, and two receiver electrodes between which a voltage is measured. There are two basic configurations: (a) in-line, whereby the four electrodes are co-linear, and (b) broadside, whereby the source dipole is parallel to the receiver dipole, and the four electrodes create a rectangle. A continuous range of configurations is possible between these extremes, since, for any source dipole, measurements could be made between any two other electrodes placed in any position.
Often, the broadside EM field components are omitted in marine electromagnetic surveying data analysis as the main part of the information of the subsurface structure can be found in the in-line electromagnetic field component. However, the inability to measure broadside EM field components reduces the uniqueness of the processed result, which is a drawback from a data analysis perspective.
Some current techniques for obtaining both in-line and broadside measurements require that the source first be towed along a line of receivers, and then be towed on a parallel course at a given offset to obtain in-line and broadside data for a single offset. Others require additional equipment, such as a reference potential line, additional receiver electrodes, broadside towing equipment, broadside electrical connections, etc., which adds complexity and cost to the system. Some current techniques anchor the EM receivers to the water bottom, requiring complex deployment and positioning systems, and making surveying of larger areas with dense receiver spacing impractical.